This baffles me. The MRS for basket 3 is clearly 2 so why should it automatically be assumed to be 4 for basket 1?

The MRS generally decreases as you go along (from left to right) a standard indifference curve. Think of the MRS at any point as backward looking. So, the MRS at basket 2 would pertain to the change from Basket 1 to 2. This would also apply to basket 3: the MRS at basket 3 would pertain to the change from Basket 2 to Basket 3.

Therefore, the MRS at Basket 2 is 20/5 = 4 and the MRS at Basket 3 is 10/5 = 2